Combination trap and baffle for high vacuum systems



March 30, 1965 n. H, HOLKEBOER ETAL. 3,175,373

COMBINATION TRAP AND BAFFLE FOR HIGH VACUUM SYSTEMS Filed Dec. 15, 1565s Sheets-Sheet 1 INVENTORS. DAVID H. HOLKEBOER FRANK PAGANO DONALD J.SANTELER WALTER F ViENNEMAN BY g va A TTORNE Y1 March 30, 1965 D. H.HQLKEBOER ETAL COMBINATION TRAP AND BAFFLE FOR HIGH VACUUM SYSTEMS 3Sheets-Sheet 2 Filed Dec. 13, 1963 m a w wmzmmmmm 03 O E v 9 mm m m w Em a 0 v" WE TW W $55.56 I ON A SE55: m HG CI. 8.83% W .M J 92,: $2 m H.H K W 0 0 g; T 5 5 N mmmm A R 0 D F D w QE BY fim om fu l M rch 30, 1965D. H. HOLKEBOER ETAL 3,175,373

COMBINATION TRAP AND BAFFLE FOR HIGH VACUUM SYSTEMS 7 Filed Dec. 13,1963 3 Sheets-Sheet 3 La 1.1 J m i H 1 nhh l f BY %w United StatesPatent 23,175,373 CGMEINATEON TRAP AND BAFlLE FGR HEGH VAQUUM SYSTEMSDavid H. Holkeboer and Frank llagano, Schenectady, Donald .l. Santeler,Scotia, and Walter F. Venneman, Schenectady, N.Y., 'assignors toAcre-Vac (Jorporation,

Troy, N.Y., a corporation of New York Filed Dec. 13, 1963, Ser. No.330,469 3i) Claims. (Cl. 62-468) This invention relates to high vacuumevacuating systems and, more particularlly, to a combination trap andbafile apparatus for use in such systems.

Where a diffusion pump employing materials such as oil or mercury isused in a system to produce a vacuum environment it has been found thatthe etliciency of the system is substantially affected by the tendencyof oil and gas molecules from the pump to make their way back into thevacuum chamber. This backstreaming, which occurs by way of molecularcollision and surface migration, contaminates the vacuum chamber andreduces the efiiciency of the pump by increasing back pressure. In thepast, attempts have been made to reduce this backstreaming byinterposing a trap having an optically tight refrigerated baffle betweenthe pump and vacuum chamber. The cold surfaces of the baffle condenseand freeze the molecules in the trap, preventing their entry into thevacuum chamber.

To date, however, those traps which have been proposed have, to ourknowledge, been only partially successful. While a bafile which rendersthe trap optically tight, i.e. no line of sight between the pump andvacuum chamber, will generally assure that most of the molecules willstrike at least one cold surface before it enters the vacuum chamber, ithas been found that a substantial number will still pass through. Thisis partially due to the inability of a single cold surface to trap andcondense a particular molecule, and partially due to the fact that, byvirtue of intermolecular collisions and oil to gas diffusion, a numberof the molecules will not strike a cold surface.

One immediate solution to this problem has been to increase thecomplexity of the baffle so that a molecule will have to strike two ormore cold surfaces of the trap. But where this has been done, it hasresulted in an undesired restriction of the flow passage with anattendant increase in back pressure and a decrease in the flowconductivity of the unit.

Still another problem which has been encountered in the design of suchunits is that caused by extreme thermal gradients in the vicinity of therefrigerated and the unrefrigerated vacuum chamber opening. For reasonswhich will be discussed more fully in the body of this application, ithas been found desirable to provide a refrigerated sleeve extendinginwardly from the vacuum chamber opening to the baffle. Because of thegradients existing, the contraction and expansion rates of theseelements will differ, and there is considerable relative movementbetween them which, if uncompensated, would weaken the structuralstrength of the unit. In the past this relative movement has beencompensated by inserting a flexible bellows section in the tube carryingrefrigerant to the baffie surfaces. However, manufacturing a refrigeranttube having such a bellows section is quite expensive, and has beenfound to be less than satisfactory.

With these and other problems in mind, we have conceived and designed acombination trap and battle in which molecules traveling anuninterrupted straight line not only will strike at least two coldsurfaces but in which the flow passage constriction is not appreciablyincreased. By other unique dimensioning features of our structure, thetendency of the molecules to avoid collision with a cold 3,l75,d73Patented Mar. 3% 1965 ICC surface, either by intermolecular collision orby oil to gas diffusion, has been minimized. By still other uniquestructural features, our structure prevents surface migration of oilmolecules. With respect to the problem of relative movement betwen thesleeve and refrigerant tubes, our structure provides a flexible mountingwhich permits relative movement between the elements without adverselyaffecting their thermal conductivity.

It is therefore an object of this invention to provide a new andimproved high vacuum system including a combination trap and baiiiehaving a double contact configuration without an appreciable restrictionof the flow path.

It is another object of this invention to provide such a systemincluding a combination trap and baffle in which the fiow path isdimensioned to minimize the backstreaming caused by intermolecularcollision and oil to gas diffusion.

It is another object of this invention to provide such a systemincluding a combination trap and bathe in which all thermal gradientareas are shielded by a cold surface to recondense transientlyevaporated condensibles.

It is another object of this invention to provide such a systemincluding a combination trap and bathe in which backstreaming into thevacuum chamber by surface migration is eliminated.

It is another object of this invention to provide such a systemincluding a combination trap and baflle which requires relatively fewstructural parts, and is reliable and relatively easy to manufacture.

In accomplishing the aforementioned objects, a principal feature residesin the provision of a container having a first opening adapted forattachment to a vacuum chamber and a second opening adapted forattachment to an evacuating means, such as an oil or mercury diffusionpump. A baffie having a plurality of first refrigerated trappingsurfaces of conventional design is mounted in the container to extendacross the first opening to block the line of sight between theopenings. A second refrigerated trapping surface extends along the lineof flow through the container and is positioned with respect to thebaffle so that no point on either the first or second refrigeratedsurfaces has line of sight contact with more than one unrefrigeratedopening. Depending upon the structure of the surfaces in the bafiie andthe positioning of the openings, the second refrigerated surface may bepositioned on one side or both sides of the baffle. The spacing betweenthe baffle and the second refrigerated surface is substantially largerthan that between adjacent trapping surfaces of the baffle so that twofiow regions having significantly different mean dimensions areprovided.

In the preferred construction of the invention, the first and secondopenings are at right angles to each other, and a reservoir of liquefiedgas, such as liquid nitrogen, is used to refrigerate the trappingsurfaces. A further feature resides in the provision of additionalrefrigerated surfaces in connection with a barrier isolating the vacuumchamber from the traps interior except through the baffle passage. Thesesurfaces are a pair of coaxial sleeves, the outer one of which isconnected to the container at the vacuum chamber opening to form thebarrier. The sleeves extend inwardly and are connected to the bafile bya number of flexible tabs which permit relative movement between thesleeves and the baifie While maintaining thermal conductivity betweenthe elements. The surfaces freeze any migrating oil particles, and thebarrier prevents their egress except over the surfaces. All surfaceswhich are subject to a temperature change so that a molecule condensedon it might boil off are either hidden from the main flow path or areprovided with still another refrigerated surface extending over it whichrecondenses the boiled off molecule.

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description taken inConjunction with the attached drawing in which:

FIGS. 13 are illustrative of conventional trap and baffle structures;

FIGS. 4-7 are schematic representations of trap and baffle structuresaccording to the present invention;

FIG. 8 is a graphic representation of the backstreaming rates throughthe two trapping regions as a function of pressure with the netbackstreaming resulting.

FIG. 9 is a schematic illustration of a high vacuum system employing acombination trap and bathe according to the present invention;

FIG. 10 -is a front view of a combination trap and baffle constructedaccording to the present invention;

FIG. 11 is a side sectional view of the trap and bafile of FIG. 10 takenalong the lines 1111;

FIG. 12 is a top sectional view of the trap and baffle taken along thelines 1212 of FIG. 11 and FIG. 13 is a fragmentary side view showing theconnection between the vacuum chamber opening sleeve and the baffletaken along lines 13-13 of FIG. 10.

In a high vacuum system employing a diffusion pump there is a tendencyfor some of the pump fluid molecules to work their way back into thevacuum chamber. This backstreaming is undesirable for several reasons,some of the most important being contamination of the vacuum chamber,impairment of the vacuum in the chamber, and an increase in pump backpressure. Some of the 'backstreaming occurs as a result of gaseousmolecular flow, while some of it is the result of surface migration ofcondensed molecules.

One solution which has been proposed to alleviate this problem is theinsertion of a trap having a refrigerated baffle in the flow pathbetween the pump and the vacuum chamber. This baffle interrupts anystraight line path by which a molecule may pass through the trap regionand presents a refrigerated surface upon which a gaseous molecule maycondense and freeze, thus preventing its passage. FIG. 1 illustrates oneconventional type of battle 1 in which a plurality of surfaces 2 and 2'are disposed in a chevron or V arrangement and extend across flow path 3of a trap 4. Molecules are prevented from bypassing baffle 1 by abarrier 5 which isolates the inlet and outlet regions.

An alternative arrangement of conventional baffle structure is shown inFIG. 2 in which a baflle 6 consists of a plurality of spaced overlappingsurfaces 7 which extend perpendicularly to flow path 8 and are connectedby an additional pair of surfaces 9 which extend in the same directionas flow path 8.

While these types of baflles are partially effective, it is known that acertain percentage of molecules will not condense and freeze upon asingle collision with a refrigerated surface. In addition, anotherpercentage of molecules will not strike any cold surface, either becauseof collision with other molecules, or because of the erratic pathscaused when a fluid diffuses into its gaseous state. This type ofdiffusion is likely to occur because of the pressure differentialbetween the vacuum chamber and the pump region. It has been found that asimple optically tight geometry of the type illustrated in FIGS. 1 and 2has only partial success in preventing passage by these three phenomena,and where an extremely high vacuum is desired, the presence of only afew molecules in the vacuum chamber can be very critical.

One solution which has been proposed has been to increase the incidenceof collision with a cold surface by increasing the complexity of thebattle to a point where a molecule traveling an uninterrupted straightline path will strike at least two cold surfaces. This type ofarrangement is illustrated in FIG. 3 where the chevron type baffie ofFIG. 1 is modified by adding a third set of surfaces 2 so that surface 2is in effect a surface common to two oppositely disposed chevronarrangements 10 and Ill. With this arrangement, no point on baffle 12has line of sight contact with more than one trap opening, and this typeof configuration defines a double contact bafile.

This more complex type of baffle decreases the likelihood of a moleculespassing through without striking any refrigerated surface, but at thesame time the added restriction in the flow path increases the backpressure and has a significant effect on the efficiency of the system.

Unlike the configurations heretofore described, the baffle arrangementaccording to our invention achieves this double contact result withoutadversely affecting system efficiency, and in addition produces furtheradvantages, which will be subsequently described, which the conventionalbaffle, either single or double contact, does not offer.

In constructing a trap according to the invention, we have provided aconventional bafile extending across the flow path which has a pluralityof first refrigerated surfaces arranged to present an optically tightgeometry. We have then added a second refrigerated surface which extendsin the direction of the flow path. By positioning this second surfacewith respect to the baffle so that no point on either the first orsecond surfaces has line of sight contact with more than oneunrefrigerated opening in the trap, a double contact configuration isachieved, but since the second surface extends along the direction offlow, it does not impede the flow, and there is no undesired addedconstriction in the trap. It should be pointed out at this point that inthe sense used herein opening refers to what could be considered eitherthe inlet or outlet of the trap. However, since there is a flow from thevacuum chamber to the pump, and an opposite flow, or backstreaming, fromthe pump to the vacuum chamber, it is not practical to describe any oneopening as being either an inlet or outlet. Instead, the openings willbe referred to as vacuum chamber or pump where a distinction between thetwo is required.

FIGS. 4-7 illustrate several ways in which the configuration accordingto our invention can be achieved; FIGS. 4 and 5 show traps of the typeillustrated in FIGS. 1 and 2, while FIGS. 6 and 7 are traps of adifferent design.

Referring now to FIG. 4, it is seen that trap 13 has a flow path 14interrupted by a simple refrigerated chevron or V baffle 15 extendingacross it. The open ends of in dividual baffie elements 16 face to theright, and it can be seen that points a and b on the interior surfacesof elements -16 would have line of sight contact with both openings. Inorder to block these points off, a second refrigerated surface ispositioned on the right hand side of trap 13 and extends in thedirection of flow path 14, along both sides of baffle 15. Surface 17extends along fiow path 14 following the interior configuration of trapif: until it intersects the plane 18 in which left hand element 16' liesand consequently intersects the planes of all other elements 16. Thus inthe example shown where the interior configuration of trap 43 iscylindrical, surface 17 takes the form of a cylinder truncated by planes18. Of course, surface 17 may take the form of a complete cylinder ifdesired.

A similar result is achieved in FIG. 5 with a different form of bathestructure. in this case bafile 19 is made up of overlapping spacedparallel plates 2%} with inner walls 21 extending in the direction offlow path 22. As can be seen, points 0 on walls 21 would have visualcontact with both openings and it is thus necessary to extend a secondsurface '23 from both sides of baffle 19 which will extend completelyaround the interior surface of trap 24 which is conventionallycylindrical. However, as was done with trap 13 of FIG. 4, surface 23 maybe partially truncated beyond lines a extending from points 0.

Another modification is shown in FIGS, 6 and 7 in which the trapopenings are at right angles to each other. In FIG. 6, chevron or Vbaffle extends across both openings 26 and 27 at a 45 angle so that eachleg 28 of an individual bathe element 29 extends at right angles to itsrespective flow path. A right angled cylindrical second surface 31extends from bathe 25 towards openings 26, 27 and terminates at theintersection with planes 2 and f in which legs 28' of bottom batheelement 29 lie.

In FIG. 7, chevron or V bathe 33 is positioned in trap 34 with its majoraxis (defined by a line drawn through the apex of individual baffleelements 35) extending at right angles to the longitudinal axis of firstopening 36 and parallel to the longitudinal axis of second opening 37.Second surface 38 is in the form of a fragmentary partially closedcylinder having a top 39 and a side 40 which extends around the interiorof trap 34 slightly overlapping the free ends of inner legs 41 of baffleelements 35 which are pointed upwardly so that their planes intersectsurface 38. It should be noted that while top 39 and side 40 of secondsurface 38 are at right angles, they both extend in the direction of theflow path by virtue of the change in flow direction caused by theangular relationship between openings 36 and -37.

Further reduction in the backstreaming rate is achieved in the trapsillustrated by virtue of the fact that each trap has in effect twotrapping regions with significantly different dimensions. One region isdefined by the mean distance between the second refrigerated surface andthe refrigerated baffle, and the second region is defined as thedistance between the adjacent refrigerated surfaces of the bathe. As wasmentioned earlier, one method by which pump fluid molecules will escapecondensation on a cold surface is where they are intercepted anddeflected by another molecule before they can strike such a surface. Theaverage distance which a molecule will travel before it will collidewith another molecule is inversely proportional to pressure, ie, at lowpressure the mean distance or free path will be longer than it is athigh pressure. However, where the pressure is high, the backstreaming isoffset by the tendency of the residual gas molecules being pumped fromthe vacuum chamber to sweep the pump fluid molecules out of the trap.Similarly, at low pressure, backstreaming is offset by the tendency of apump fluid molecule to strike a cold surface before it collides withanother molecule.

As is illustrated by the graph of FIG. 8, it has been found that thecollision rate, and hence the backstreaming caused by the phenomenonwill reach a peak at an intermediate pressure where the mean free pathis of the same order or size of its trap region dimensions. Thus, byproviding two trapping regions A and B (shown in FIG. 11) havingsignifiicantly different dimensions it is possible to minimize theeffect of a peak backstreaming rate in one region by the significantlylower backstreaming rate in the second region. Conversely, where thebackstreaming rate reaches its peak in the second region, it will beoffset by the lower backstreaming rate in the first region. Since thetrapping effect of both regions is cumulative, it is possible to providea net backstreaming rate which is lower than that provided by a singletrapping region and which is not adversely affected by pressurevariations.

Ideally, the dimensional difference between the trapping regions shouldbe as large as possible. Practically it has been found that a meandistance of between five and ten inches between the second surface andthe bathe, and a mean distance of one-half inch between adjacentsurfaces of the bathe has been satisfactory.

A high vacuum system in which applicants invention may be utilized isillustrated in FIG. 9 in which a vacuum chamber 42 is initiallyevacuated by a mechanical pump 43. When the limit of pump 43 is reached,a diffusion pump 44 employing a suitable medium as oil or mercury isthen switched in for the final evacuation process. A combination trapand bathe 45 is interposed between cham- 5 her 42 and diffusion pump 44and operates in the manner heretofore and subsequently described.

A specific design of a combination trap and bathe according to ourinvention is illustrated in FIGS. 10-13 and embodies additional featureswhich add to its effectiveness.

Trap 46 illustrated in FIGS. 10-13 employs a two collision configurationsimilar to that shown in FIG. 7, and comprises a cylindrical casing 47of stainless steel having a first opening 48 in its side fitted with asleeve 49 and a flange 59 adapted for attachment to a vacuum chamber. Asecond sleeve 52 extends inwardly from opening 48 and is Welded tosleeve 49 via a sealing ring 53. A third sleeve 54, coaxial with, but oflesser diameter than, sleeve 52 extends inwardly from opening 48. Bothsleeves 52 and 54 are bolted to a rectangular plate 64. Plate 64 has acentral opening 65 which provides access to a bathe 55 extending acrossopening 48 and a pair of flanges 66 extending around bathe 55 to providean optically tight passage from the vacuum chamber to the interior oftrap 46. The assembly of sleeves 52 and 54 and plate 64 is thermallyconnected to bathe 55 by a number of twisted tabs 67 which form aflexible coupling.

Bathe 55 consists of a plurality of pairs of vertically spaced planarsurface elements 56 and 57 extending across opening 48 and connected attheir ends by mounting plates 68. The major surfaces of elements 56 and5'7 lie in intersecting planes at an angle to the horizontal. The row ofelements 56 is slightly vertically offset from the row of elements 57 sothat the elements of each pair are spaced to form an incomplete chevronor V. However, each element 56 slightly overlaps its adjacent element 57so that a substantially chevron or V arrangement is obtained, and theconfiguration is optically tight. This substantially V configuration hasbeen found to be preferable to the completely closed V because, whilemaintaining optical tightness, it provides a more conductive flow pathby reducing the probability that a molecule will be deflected back intothe vacuum chamber rather than into the trap interior.

Each of the elements 56 and 57 is mounted on a pair of hollow verticaltubes 53 which is attached to a refrigerant reservoir 59 mounted in thetop of casing 47. Tubes 5% supply refrigerant to elements 56 and 57 andchill them as well as sleeves 52, 54 by conduction.

Reservoir 59 extends inwardly into casing 47 with a slight spacing fromits interior wall. In the trap illustrated, liquid nitrogen is used asthe refrigerant agent, but any similar cooling medium capable ofcondensing and freezing pump fluid molecules may be employed. The bottom60 of reservoir 59 forms part of the second tr apping surface inconjunction with a semi-cylindrical shroud 61 which is welded at one endto the side wall of reservoir 55 and extends into casing 47 with itsconcave surface facing bathe 55 and extending parallel to the directionof flow. Shroud 61 is conductively refrigerated by the liquified gas inreservoir 59 and extends around the interior wall of casing 4-7 until itslightly overlaps bathe 55. Inner elements 56 of bafhe 55 are pointedupwardly so that the plane of their major surfaces intersect theelements 60, 61 of the second trapping surface.

In the embodiment illustrated a second bathe 62 extends across opening5i. Unlike baffle 55, baffle 62 is water cooled, or otherwiserefrigerated, by pipe 69 leading to a fluid source (not shown) and maybe inserted in the trap to perform an initial condensing operation onfluids escaping from the diffusion pump as well as providing a thermalbarrier between the pump and the trap interior. Addition of bathe 62reduces nitrogen consumption and, more importantly, reduces the quantityof fluid condensed and frozen in the trap interior. This result isdesirable since a build up of fluid on the trapping surfaces will tendto insulate them, reducing efliciency. Because of a lower a 3,1 avebuild up of pump fluid, the system can be run for longer eriods of timewithout the necessity of shutting down for purposes of cleaning,

An additional problem which contributes to bachstreaming is theevaporation of condensed molecules when the surfaces upon which they arecondensed are subject to warming. This occurs in those sections of thetrap where one end of a surface is connected to a refrigerated source,and the other end is connected to an unrefrigerated source. Theresulting thermal gradient along the surface is such thatthe condensingpoint will be constantly shifting, depending upon the temperaturedifferential between the sources. In the trap illustrated in- FIGURES-13, this occurs in two principal areas. When reservoir 59 is filled theimmediately adjacent portion of casing 47 will be chilled to a pointwhere it will condense gas on its surface. However, as the nitrogen evellowers, the area in the vicinity will warm, and the gas condensedthereon will tend to boil off. More significantly, because of itslocation adjacent the vacuum chamber, one end of second sleeve 52 isconductively refrigerated by its attachment to plate 64, while itsopposite end is connected to sleeve 49 which is unrefrigerated. Wheregas has condensed on this surface, a slight shift in the thermalgradient along sleeve 52 may cause rapid boil 01f and diffusion into thevacuum chamber.

In both these cases a second refrigerated surface is provided which isslightly spaced from the thermal gradient surface and which will trapand refreeze the boiled off molecule. With respect to side wall 63 thisis accomplished by the side walls of reservoir 59. Sleeve 52 isprotected by third sleeve 54 which is conductively refrigerated by itsthermal connection with bafile 55, but is not connected to any warmersource at its free end.

As was indicated earlier, the thermal gradient in the region of baffle55 and second and third sleeves 52 and 54- raises the problem ofrelative movement caused by unequal expansion and contraction. In thetrap illustrated, this problem has been overcome by independentlyrigidly mounting bafiie 55 and sleeves 52 and 54 and then thermallyconnecting them with a plurality of tabs 67 which are twisted to providea flexible connection. The optical tightness of the assembly ismaintained by plate (54 to which sleeves 52 and 54 are attached andwhich has a pair of flanges 66 extending around mounting plates 68.

It should also be noted that sealing ring 53 blocks passage of any gasexcept through baffie 55, and the combination of refrigerated surfaces60, 61, baffle 55 and third sleeve 54 provide extended refrigeratedsurfaces over which a migrating molecule must pass before entering thevacuum chamber. These surfaces virtually eliminate the possibility ofsurface migration.

The trap of FIGS. 1013 also has the significantly different trappingregion dimensions previously described. As can be seen from thedrawings, it is apparent that the distance B between surfaces 60, 61 andbatlle 55 is substantially greater than the distance A betweenvertically adjacent elements of baffle 55.

While certain preferred embodiments of the invention have been describedand illustrated, it is to be understood that the invention is notrestricted soley thereto, but that it is intended to cover allmodifications which would be apparent to one skilled in the art, andwhich come within the spirit and scope of the invention.

We claim:

1. For use in a high vacuum system, a combination trap and baffieapparatus comprising a casing having a first opening adapted forattachment to a vacuum chamber and a second opening adapted forattachment to an evacuating means, a bafiie having first refrigeratedtrapping means mounted in said casing to block the line of sight betweensaid openings, and second refrigerated trapping means mounted in saidcasing. said second trapping means having major dimensions extendingalong the direction of flow through said openings, any point on saidsecond trapping means having line of sight contact with no more than oneof said openings.

2. Combination trap and baffie apparatus according to claim 1 whereinsaid second trapping means has line of sight contact with only a portionof said first trapping means.

3. Combination trap and baffie apparatus according to claim 1' whereinany point on said first trapping means has line of sight contact with nomore than one of said openings.

4. Combination trap and bafile apparatus according to claim 1 whereinsaid first trapping means comprises a first and second plurality ofplanar surfaces extending across at least one of said openings, saidfirst and second planar surfaces forming a substantially V section; andsaid second trapping means has a surface intersecting the planes of atleast said first plurality of planar surfaces.

5. Combination trap and baffle apparatus according to claim 4 whereinthe mean distance between said first trapping means and said secondtrapping means is substantially greater than the mean distance betweenadjacent ones of said V sections.

6. Combination trap and baffle apparatus according to claim 4 whereinsaid first and second openings are at an angle to each other.

7. Combination trap and bafiie apparatus according. to claim 1 whereinsaid first trapping means comprises a first and second plurality ofplanar surfaces extending across at least one of said openings, saidfirst and second planar surfaces forming a substantially V section; andsaid second trapping means has surfaces intersecting the planes of saidfirst and second plurality of planar surfaces.

8. Combination trap and bafiie apparatus according to claim 7 whereinthe mean distance between said first trapping means and said secondtrapping means is substantially greater than the mean distance betweenadjacent ones of said V sections.

9. Combination trap and baffle apparatus according to claim 7 whereinsaid first and second openings are at an angle to each other. I

10. Combination trap and bathe apparatus according to claim 9 whereinsaid first and second planar surfaces extend across both of saidopenings.

11. Combination trap and baffle apparatus according to claim 1 whereinsaid first and second openings are in line with each other; said firstrefrigerated trapping means comprises a plurality of spaced surfacesextending partially across the flow path through said container, saidsurfaces partially overlapping each other whereby the line of sightbetween said first and second openings is blocked; and said secondrefrigerated trapping means comprises a surface extending along saidflow path on either side or" said first trapping means.

12. Combination trap and bafiie apparatus according to claim 11 whereinthe mean distance between said first trapping means and said secondtrapping means is substantially greater than the mean distance betweenadjacent ones of said spaced surfaces.

13. Combination trap and bafile apparatus according to claim 11 whereinsaid surface of said second refrigerated trapping means extends fromeither side of said first trapping means a sufficient distance tointersect an uninterrupted straight line drawn from any interior pointof said first trapping means otherwise having line of sight contact withboth the said openings.

14. Combination trap and baffle apparatus according to claim 1 whereinthe mean distance between said first trapping means and said secondtrapping means is substantially larger than the mean distance betweenelements of said first trapping means.

15. Combination trap and baffle apparatus according to claim 1 whereinsaid first trapping means comprises a plurality of spaced planar surfacepairs extending across at least one of said openings, the elements ofsaid pairs lying in intersecting planes but spaced from each other sothat said elements form an incomplete V configuration.

16. Combination trap and baffle apparatus according to claim whereinsaid elements of said pairs overlap each other to provide an opticallytight arrangement.

17. For use in a high vacuum system, a combination trap and batllecomprising a container having a first opening adapted for connection toa vacuum chamber and a second opening at right angles to said firstopening adapted for connection to an evacuating means, the space betweensaid first and second openings defining a flow path; a baffle in saidflow path extending across said first opening to block the line of sightbetween said first and second openings, said baffle including firstrefrigerated trapping surfaces for condensing and trapping gaseousparticles tending to flow into the vacuum chamber; a second refrigeratedtrapping surface extending along the line of flow between said first andsecond openings; and means for refrigerating said first and secondtrapping surfaces.

18. Combination trap and baffle apparatus according to claim 17 whereinsaid first refrigerated trapping surfaces comprise a plurality of spacedpairs of planar surface elements, each mounted on a tube connected tosaid refrigerating means and forming a substantially V section bafilewhereof the plane of each element surface intersects said secondrefrigerated trapping surface.

19. Combination trap and baffie apparatus according to claim 18 whereinthe planar surface elements of said pairs are transverse to said fiowpath to and from said openings, respectively.

20. Combination trap and baffle apparatus according to claim 18 whereinthe planar surface elements of said pairs are in an overlapping spacedrelationship whereby an incomplete V is formed.

21. Combination trap and baffle apparatus according to claim 17 whereinsaid means for refrigerating comprises a liquified gas reservoir havinga flow tube in connection with said first refrigerated trappingsurfaces, and said second refrigerated trapping surface is in thermalconnection with said reservoir.

22. Combination trap and battle apparatus according to claim 21 whereinsaid liquifled gas reservoir is mounted in said container and theexternal surface of said reservoir augments said second refrigeratedtrapping surface.

23. Combination trap and bafiie apparatus according to claim 17 whereinsaid second refrigerated trapping surface comprises a semicylindricalsheet having its interior surface facing towards said bafile.

24. Combination trap and baffle apparatus according to claim 17 furthercomprising a second refrigerated bafile extending across said secondopening.

25. Combination trap and battle apparatus according to claim 24 whereinsaid second bafile extending across said second opening is maintained ata higher temperature than said first and second trapping surfaces.

26. Combination trap and battle apparatus comprising a container havinga first opening adapted for connection to a vacuum chamber and a secondopening at right angles to said first opening adapted for connection toan evacuating means, the space between said openings defining a flowpath; a first sleeve rigidly attached to said container and extendinginwardly into said container from said first opening; a second sleevecoaxial with and of lesser diameter than said first sleeve extendinginto said container and rigidly attached at its inner end to said firstsleeve; a bafile mounted in said container interiorly of said sleevesextending across said first opening to block the line of sight betweenthe vacuum chamber and the interior of said container; thermallyconductive 10 flexible means connecting said baffie with said first andsecond sleeves to permit relative movement between said bafile and saidsleeves caused by unequal contraction and expansion; and means torefrigerate said batlle and said sleeves.

27. Combination trap and baffle apparatus according to claim 26 whereinthe interior ends of said sleeves are mounted on a plate extendingacross said first opening, said plate having flanges surrounding theends of said baffle and said thermally conductive flexible meanscornprise a plurality of flexible metallic tabs attached to said plateand said baffle.

28. Combination trap and battle apparatus comprising a container havinga first opening adapted for connection to a vacuum chamber and a secondopening at right angles to said first opening adapted for connection toan evacuating means, the space between said openings defining a flowpath;

a first sleeve rigidly attached to said container and ex tendinginwardly into said container from said first opening;

a second sleeve coaxial with and of a lesser diameter than said firstsleeve extending into said container;

a plate rigidly attached to the inner ends of said sleeves and extendingacross said first opening, said plate having an opening for passage ofgas therethrough;

a refrigerated bafile rigidly mounted in said container interiorly ofsaid plate extending across said first opening and said plate opening toblock the line of sight between said first opening and the interior ofsaid container, said baffle including a plurality of first refrigeratedtrapping surfaces comprising a plurality of vertically spaced planarsurface pairs connected by hollow tubes, the elements of said pair lyingin intersecting planes but spaced from and overlapping each other sothat a series of incomplete \ls is formed,

said plate having flanges overlying the ends of said baffle;

a plurality of flexible metallic tabs thermally connecting said bairleto said plate to permit relative movement therebetween;

a liquified gas reservoir mounted in the top of said container, saidtubes being connected to said reservoir for providing liquified gas torefrigerate said baffle and said sleeves;

and a semi-cylindrical shroud attached to said reservoir inside saidcontainer, said shroud having its concave major surface extendingtowards said second opening, the bottom of said reservoir and saidshroud surface defining a second refrigerated trapping surface, saidshroud extending around the interior of said container and overlappingthe ends of said bafiie.

29. Combination trap and bafile apparatus according to claim 28 furthercomprising a second refrigerated bafile extending across said secondopening, said second baffle being maintained at a higher temperaturethan said first and second refrigerated surfaces.

30. Combination trap and bafile apparatus according to claim 28 whereinthe mean distance between said second trapping surface and said bafileis substantially larger than the mean distance between adjacent ones ofsaid first trapping surfaces.

References Cited in the file of this patent UNITED STATES PATENTS2,934,257 Power Apr. 26, 1960 3,081,068 Milleron Mar. 12, 1963 3,103,108Santeler Sept. 10, 1963 3,122,896 Hickey Mar. 3, 1964

1. FOR USE IN A HIGH VACUUM SYSTEM, A COMBINATION ITRAP OF BAFFLEAPPARATUS COMPRISING A CASING HAVING A FIRST OPENING ADAPTED FORATTACHMENT TO A VACCUUM CHAMBER AND A SECOND OPENING ADAPTED FORATTACHMENT TO AN EVACUATING MEANS, A BAFFLE HAVING FIRST REFRIGERATEDTRAPPING MEANS MOUNTED IN SAID CASING TO BLOCK THE LINE OF SIGHT BETWEENSAID OPENINGS, AND SECOND REGRIGERATRD TRAPPING MEANS MOUNTED IN SAIDCASING, SAID SECOND TRAPPING MEANS HAVING MAJOR DIMENSONS EXTENDINGALONG THE DIRECTION OF FLOW THROUGH SAID OPENINGS, ANY POINT ON SAIDSECOND TRAPPING MEANS HAVING LINE OF SIGHT CON TACT WITH NO MORE THANONE OF SAID OPENINGS.